A CMOS imager includes a focal plane array of pixels, each pixel including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. A readout circuit is provided for each pixel and includes at least a source follower transistor and optionally, a row select transistor for coupling the source follower transistor to a column output line. The pixel also typically has a floating diffusion region, connected to the gate of the source follower transistor. Charge generated by the photosensor is sent to the floating diffusion region. The imager may also include a transistor for transferring charge from the photosensor to the floating diffusion region and another transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel, for example a four transistor pixel, perform the necessary functions of (1) photon to charge conversion; (2) transfer of charge to the floating diffusion region; (3) resetting the floating diffusion region to a known state before the transfer of charge to it; (4) selection of a pixel for readout; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo converted charges. The charge at the floating diffusion region is converted to a pixel output voltage by a source follower output transistor.
A schematic diagram of a conventional CMOS four-transistor (4T) pixel 20 is illustrated in FIG. 1. The four transistors include a transfer transistor 22, a reset transistor 23, a source follower transistor 24, and a row select transistor 25. A photosensor 21, e.g., a pinned photodiode, converts incident light into charge. A floating diffusion region 26 receives charge from the photosensor 21 through the transfer transistor 22 (when activated) and is also connected to the reset transistor 23 and the gate of the source follower transistor 24. The source follower transistor 24 outputs a signal proportional to the charge accumulated in the floating diffusion region 26 to a sampling circuit when the row select transistor 25 is turned on. The reset transistor 23 resets the floating diffusion region 26 to a known potential prior to transfer of charge from the photosensor 21. The photosensor 21 may be a photodiode (as shown in FIG. 1), a photogate, or a photoconductor. If a photodiode is employed, the photodiode may be formed below a surface of the substrate and may be a p-n-p photodiode, an n-p-n photodiode, a p-n photodiode, or a n-p photodiode, among others.
CMOS semiconductor imaging devices include an array of pixels such as pixel 20 of FIG. 1, which convert light energy received, through optical lenses, into electrical signals. The electrical signals produced by the array of pixels are processed to render a digital image.
The amount of charge generated by the photosensor 21 corresponds to the intensity of light impinging on the photosensor 21, for a given integration time. Accordingly, it is important that all of the light directed to the photosensor 21 impinges on the photosensor 21 rather than being reflected or refracted toward another photosensor (known as optical crosstalk).
For example, optical crosstalk may exist between neighboring photosensors in a pixel array. In an ideal imager, light enters only through the surface of the photosensor that directly receives the light stimulus. In reality, however, some light intended for one photosensor also impinges on another photosensor through the sides of the optical path existing between a lens and the photosensor.
Optical crosstalk can bring about undesirable results in the images produced by the imager. The undesirable results can become more pronounced as the density of pixels in the imager array increases, and as pixel size correspondingly decreases. The shrinking pixel sizes and greater pixel density make it increasingly difficult to properly focus incoming light on the photosensor of each pixel without accompanying optical crosstalk.
Optical crosstalk can cause a blurring or reduction in contrast in images produced by the imager. Optical crosstalk also degrades the spatial resolution, reduces overall sensitivity, causes color mixing, and leads to image noise after color correction. As noted above, image degradation can become more pronounced as pixel and related device sizes are reduced. Furthermore, degradation caused by optical crosstalk is more conspicuous at longer wavelengths of light. Light having longer wavelengths penetrates more deeply into the silicon structure of a pixel, providing more opportunities for the light to be reflected or refracted away from its intended target photosensor.
Electrical crosstalk may also occur when the photogenerated signals migrate through the silicon between pixels, and are collected at the wrong photodiode. Electrical crosstalk becomes more pronounced as pixel size decreases, and for longer wavelength light.
FIG. 2 illustrates the problem of optical and electrical crosstalk in a conventional frontside illuminated imager. A conventional frontside illuminated imager includes an array of pixels. For simplicity, a cross section of a single pixel 2 is illustrated. Pixel 2 has, for example, photodiodes, formed within a substrate 41. FIG. 2 also illustrates a metallization and interlayer dielectric layer 51 in contact with the substrate 41. A nitride layer 91, color filter array layer 96, and microlens 97 are also provided. Ideally, incoming light 13 should stay within a photosensor optical path 12 when traveling through a microlens 97 to a respective photosensor of the pixel 2. However, light 13 can be reflected within the respective layers of the imager and at the junctions between these layers. The incoming light 13 can also enter the pixel at different angles, causing the light to be incident on a different photosensor. Loss of the incident light 13 as it travels through the various layers also decreases the quantum efficiency of the device.
As noted, electrical crosstalk may also occur between pixels when photogenerated electrons migrate through the silicon layers. The thicker the silicon layers are, the greater space and opportunity for such migration to occur. However, thicker silicon layers provide greater overall structural stability to a device containing a pixel array.
Accordingly, there is a need and desire for an improved apparatus and method for reducing crosstalk and related electrical interference in imaging devices, without compromising structural stability. There is also a need to more effectively and accurately increase overall pixel sensitivity and provide improved crosstalk immunity without adding complexity to the manufacturing process and/or increasing fabrication costs. There is also a need to increase quantum efficiency. It would further be beneficial to provide an imager device having wafer level packaging.